1. Field of the Invention
The present invention relates to a process for producing rhombohedral system boron nitride.
2. Description of the Prior Art
A conventional process for the production of rhombohedral system boron nitride comprises mixing boric acid or borax with potassium cyanide and heating the mixture thereby obtained. In this process, the rhombohedral system boron nitride is obtained as a mixture with hexagonal system boron nitride. The two boron nitrides are alike in their chemical properties and specific gravities, and accordingly it has been impossible to isolate the rhombohedral system boron nitride from the mixture.
The rhombohedral system boron nitride is susceptible to high pressure phase transition by impact pressure and thereby readily converted into cubic system boron nitride. On the other hand, the hexagonal system boron nitride is converted into wurtzite structure boron nitride. The wurtzite structure boron nitride is inferior in its properties as grinding material to the cubic system boron nitride, since its hardness is low and sharp corners are hardly obtainable by cleavage. Thus, the boron nitride containing a great amount of hexagonal system boron nitride is inferior to pure rhombohedral system boron nitride, as a material for the production of high pressure type boron nitride for grinding.
Such inclusion of a substantial amount of hexagonal system boron nitride by the above-mentioned process is caused as follows. In the formation of boron nitride, the higher the reaction temperature becomes, the more readily, hexgonal system boron nitride tends to form. Therefore, the production of the rhombohedral system boron nitride is carried out at a relatively low temperature. Under such a low temperature condition, a pair of boric acid reacts with potassium cyanide to form potassium borate, which will then remain in a molten state in the reaction system. When borax is used as the starting material, the borax and potassium borate will form a mixed molten solution, which will then remain in the reaction system. These alkali metal borates facilitate the conversion of the boron nitride to hexagonal system boron nitride. Thus, the presence of the alkali metal borates gives rise to the formation and inclusion of the hexagonal system boron nitride.